The overall goal of this proposed research is to investigate the development of the brain-pituitary-adrenal axis in the mammalian fetus. This includes determining: (1) if the drive to ACTH secretion increases during gestation and (2) if there is an increase in the ratio of bioactive to immunoreactive ACTH in fetal plasma close to term which correlates with the increase in fetal plasma cortisol occurring during this time. The chronically cannulated lamb fetus is the animal model chosen for study. It is possible to study brain-pituitary-adrenal relationships in this model in the absence of complications caused by the presence of anesthesia and surgical trauma, both of which are inherent in experiments on acutely prepared animals. The information derived from these proposed studies will further our understanding of mechanisms regulating basal activity in an endocrine system necessary for preparing the fetus for birth and in some species, necessary for the initiation of labor. Increasing our understanding of the mechanisms determining the patterns of change in basal fetal plasma cortisol and ACTH levels during the latter part of gestation are essential for furthering our knowledge of the development processes which culminate in the birth of a healthy infant.